A typical loudspeaker is an electro-dynamic transducer attached to a diaphragm of some depth, diameter and shape. Electro-dynamic describes a transducer that moves back and forth in response to an alternating voltage source to stimulate adjacent air molecules. Some of these types of loudspeakers may be considered a commodity and are inexpensive. They are typically mounted on a baffle as part of an existing product or structure; in some form of housing for practical containment or in some cases a specialized enclosure is utilized to enhance the bass performance.
One problem with these types of loudspeakers is that the driver may have a favorable acoustic impedance only over a narrow range of frequencies depending on its size. The smaller driver generally has unfavorable acoustical impedance for lower frequencies and vise versa for larger ones. The enclosure also favors a narrow range of frequencies and for other frequencies it may react violently creating a plethora of incoherent internal standing waves that modulate the diaphragm with nonsymmetrical vibration patterns. These random internal modulations disturb the natural dispersion pattern of the driver and cause electrical feedback (reactance) to the amplifying source. Brute force power and heavy gauge wiring are current attempts to minimize this problem for the amplifier and the effects on sound quality.
Another problem is the general acoustic impedance differential that exists on either side of the driver diaphragm. The diaphragm must work simultaneously in two different acoustic environments as the enclosure creates standing waves that constantly modify the drivers' acoustic impedance in most of its frequency range. Reflected waves from the room cause additional modifications of the drivers' acoustic impedance more as the frequencies go lower towards that of the rooms' dimensions. Smaller enclosures can be worse because of the even higher frequencies that are reflected internally and the lack of low frequency capabilities.
Two identical drivers will sound different due to their operating enclosure. One solution with mid-range speakers is to produce units with a solid basket behind the diaphragm. This may prevent random standing waves from interfering with the other drivers but it may create extreme backpressure for the range of frequencies produced by the midrange driver. This causes the driver to see a distinct acoustic impedance differential throughout its operating range thereby preventing it from producing a natural sound.
Loudspeaker driver dimensions favor a certain range of frequencies thus making a single size for all frequencies difficult if wide axis listening is desired. It is a design goal to produce loudspeakers of the smallest dimensions necessary at minimum cost while maintaining the proper loudness level while retaining the sonic presentation of full frequency range, low distortion and wide-constant dispersion. A solution is the use of multiple drivers operating for a common acoustic purpose. This is reflected in current loudspeaker designs in an effort to produce subjectively accepted loudspeakers.
When a single driver is used, it is typically designed to favor lower or higher end frequencies while attempting to maintain quality in the middle ranges. The human ear tends to be more sensitive to the higher frequencies but the human ear-brain combination prefers to hear all of the frequencies in the spectrum without phase or frequency aberrations to interrupt the flow of energy of the event otherwise it will appear to be artificial. The reproduction of sound is typically for either of two purposes and that is communication and entertainment. The latter requires unencumbered sonic balance and dispersion to balance the energy in the listening environment.
The continued efforts to perfect sound reproduction with predictable field results depend greatly on a solution to solve the dilemma of the enclosure. Engineers recognize the drivers' enclosure as a design challenge. The use of the apparatus as explained in the pending application can improve sound quality.